1. Field of the Invention
The present invention relates generally to angular position indicating devices, and more particularly, to a capacitive shaft encoder for providing an analog signal corresponding to the angular position of a shaft.
2. Description of the Prior Art
Heretofore, capacitive shaft encoders have employed an AC voltage source for excitation because of the availability of such excitation circuits. Although such voltage sources typically generate a signal having a precise frequency, it is difficult to accurately control the signal amplitude. This presents a problem when the source is applied to a capacitive divider network, since the output voltage of the network is proportional to the amplitude of the applied signal. Consequently, variations in the amplitude of the generated signal introduce errors into the output voltage of the encoder.
Examples of capacitive shaft encoders employing an alternating current voltage source are found in the U.S. Pat. Nos. to Higinbotham et al, 2,548,790; Wolfendale, 3,296,522; Eide et al, 3,783,374; and Melnyk, 3,702,467. Each of these devices has the disadvantage that the amplitude of the signal applied to a capacitor divider network may vary in amplitude, thus introducing errors into the output voltage.
In other related prior art, the Stick U.S. Pat. No. 3,760,392, teaches a capacitive shaft encoder which provides a digital output, and Bourgonnier et al, U.S. Pat. No. 2,659,039; Pihl, U.S. Pat. No. 2,847,640; and Lippel, U.S. Pat. No. 3,222,668, disclose various types of capacitive transducers suitable for use in shaft encoding devices.